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Subject: Re: [PATCH v3] ACPI/processor_idle: Remove dummy wait if kernel is in
 guest mode
To:     David Laight <David.Laight@ACULAB.COM>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
        "rjw@rjwysocki.net" <rjw@rjwysocki.net>,
        "lenb@kernel.org" <lenb@kernel.org>
References: <20191023074945.17016-1-fengwei.yin@intel.com>
 <30ee0a348f624698801691f65eeecd87@AcuMS.aculab.com>
From:   Yin Fengwei <fengwei.yin@intel.com>
Message-ID: <9bb3baef-b23e-a383-d50f-f795033c0c02@intel.com>
Date:   Thu, 24 Oct 2019 09:22:18 +0800
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On 2019/10/23 下午4:45, David Laight wrote:
> From: Yin Fengwei
>> Sent: 23 October 2019 08:50
> 
> 
>> In function acpi_idle_do_entry(), an ioport access is used for dummy
>> wait to guarantee hardware behavior. But it could trigger unnecessary
>> vmexit if kernel is running as guest in virtualization environtment.
>>
>> If it's in virtualization environment, the deeper C state enter
>> operation (inb()) will trap to hyervisor. It's not needed to do
>> dummy wait after the inb() call. So we remove the dummy io port
>> access to avoid unnecessary VMexit.
>>
>> We keep dummy io port access to maintain timing for native environment.
>>
>> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
>> ---
>> ChangeLog:
>> v2 -> v3:
>>   - Remove dummy io port access totally for virtualization env.
>>
>> v1 -> v2:
>>   - Use ndelay instead of dead loop for dummy delay.
>>
>>   drivers/acpi/processor_idle.c | 36 ++++++++++++++++++++++++++++++++---
>>   1 file changed, 33 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
>> index ed56c6d20b08..0c4a97dd6917 100644
>> --- a/drivers/acpi/processor_idle.c
>> +++ b/drivers/acpi/processor_idle.c
>> @@ -58,6 +58,17 @@ struct cpuidle_driver acpi_idle_driver = {
>>   static
>>   DEFINE_PER_CPU(struct acpi_processor_cx * [CPUIDLE_STATE_MAX], acpi_cstate);
>>
>> +static void (*dummy_wait)(u64 address);
>> +
>> +static void default_dummy_wait(u64 address)
>> +{
>> +	inl(address);
>> +}
>> +
>> +static void default_noop_wait(u64 address)
>> +{
>> +}
>> +
> 
> Overengineered...
> Just add:
> 
> static void wait_for_freeze(void)
> {
> #ifdef	CONFIG_X86
> 	/* No delay is needed if we are a guest */
> 	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> 		return;
> #endif
> 	/* Dummy wait op - must do something useless after P_LVL2 read
> 	   because chipsets cannot guarantee that STPCLK# signal
> 	   gets asserted in time to freeze execution properly. */
> 	inl(acpi_gbl_FADT.xpm_timer_block.address);
> }
> 
> and use it to replace the inl().
OK. I was trying to avoid any impact to native case.

> 
> ...
>> +#ifdef	CONFIG_X86
>> +	/* For x86, if we are running in guest, we don't need extra
>> +	 * access ioport as dummy wait.
>> +	 */
>> +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
>> +		pr_err("We are in virtual env");
>> +		dummy_wait = default_noop_wait;
>> +	} else {
>> +		pr_err("We are not in virtual env");
>> +	}
>> +#endif
> 
> WTF are the pr_err() for???
Sorry. Didn't remove my debug code...

Regards
Yin, Fengwei

> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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